In a conventional cassette tape recording and/or reproducing device, signals are recorded on and reproduced from a magnetic tape held within a cassette which placed in or on a cassette holder of the device. The cassette holder is frequently a tape receiving recess and the device includes a tape transport system having two reel drive spindles which project into the recess. The cassette correspondingly incorporates a take-up hub and a supply hub adapted to receive respective ones of the drive spindles. The drive spindles may be rotated either clockwise or counter-clockwise to rotate the hubs and thereby effect transfer of the tape from one hub to the other depending on the direction of rotation.
The recording and reproducing of signals on the tape within the cassette is carried out by means of magnetic heads whose construction is well known. The magnetic heads, which may include record heads, reproduce heads, erase heads and the like, are mounted within the device at operative positions relative to the cassette holder and drive spindles, and the cassette has defined portions in a side wall with openings into which the respective magnetic heads extend to contact the tape when the cassette is in the cassette holder for recording and reproducing signals. All such magnetic heads acquire residual magnetism over a period of time, and unless they are demagnetized (degaussed), the residual magnetism will cause noise and distortion during recording and/or during playback.
It is well known that the application of an alternating magnetic field having flux peaks of decreasing magnitude is effective to demagnetize the heads. This type of magnetic field, although periodically varying in intensity, will be referred to in this application as a decreasing magnetic field. An alternating magnetic field having flux peaks of constant magnitude will be referred to herein as a constant magnetic field, although in actuality it is constant neither in intensity or polarity.
Various head demagnetizers for providing a decreasing magnetic field have been proposed which are constructed in a cassette-shaped housing and are adapted to be positioned in the recording and/or reproducing device in place of a tape cassette. These demagnetizers have utilized the driving power from the spindles to produce the decreasing magnetic field. One way this is done is to fix a magnetic field generator within the housing at the wall portion adjacent each operative position, thus also a "demagnetizing" position, and to electronically decrease the magnetic field, for example by discharging a capacitor. Another way is to use the drive spindles to rotate a permanent magnet and so produce the alternating magnetic field and to mechanically withdraw the rotating magnet from the demagnetizing position. Such prior art demagnetizers have tended to be relatively complicated in construction, either electrically or mechanically. Since a cassette demagnetizer is intended to be relatively inexpensive, useful over a long period of time and operated by unsophisticated consumers, it would be highly advantageous for it to have a minimum number of parts, mechanical or electrical. Such a demagnetizer, however, must still achieve a sufficient relative decrease in the magnetic field to produce the desired demagnetizing effect.